1. Technical Field
The present invention relates generally to a collision alarm system for an automotive vehicle which rases an alarm when obstacles are tracked within a detection zone defined ahead of the vehicle, and more particularly to an improvement on a collision alarm system which is designed to avoid raising a false alarm.
2. Background Art
Japanese Patent First Publication Nos. 5-166097 and 4-201643 teach a collision alarm system which issues an alarm when a distance between a vehicle and an obstacle ahead thereof falls within a dangerous distance.
The former (No. 5-166097) represents a simple improvement on a conventional collision alarm system which emits a laser beam to track a preceding vehicle and which issues an alarm when an intervehicle distance between the preceding vehicle and a system vehicle equipped with this system falls within a dangerous zone. This system however, does not take stationary obstacles such as side edges of a road into account. Alarms are therefore raised frequently, especially when a side edge of a curved road is detected during cornering, making vehicle passengers feel uncomfortable.
The latter (No. 4-201643) discloses an improved alarm system which is designed to restrict portions of a detection zone on both sides of a road for avoiding a false alarm such as that undesirably raised in the former system.
The above prior art collision alarm systems, however, have suffered from the following drawbacks.
For example, when a vehicle has entered a curved road from a straight road, a guardrail, although tracked initially as a stationary object, becomes identified as a preceding vehicle because it is viewed as an object present at a constant distance away from the vehicle after the vehicle begins turning. Thus, even when the vehicle slightly approaches the guardrail, it recognizes the guardrail as an approaching preceding vehicle that is decelerating, and a false alarm is raised to a vehicle operator.
Additionally, when a large vehicle squeezes between the vehicle and an immediately preceding vehicle, a long side thereof will be tracked sequentially from the front to the rear so that it is identified as an approaching decelerating vehicle, thereby issuing a false alarm.